Integrated circuit buffering solutions considering sink delays

ABSTRACT

Optimizing timing in a VLSI circuit by generating a set of buffer solutions and determining a most critical delay and a sum of critical delays for each solution in the set of solutions. Quantifying a relationship between the most critical delay and the sum of critical delays for each solution. Comparing each solution&#39;s quantified relationship to the quantified relationship of each other solution in the set of solutions. Identifying, based on the comparing of each solution&#39;s relationship to the relationship of each other solution in the set of solutions, at least one solution in the set of solutions to have a worse relationship between the most critical delay and the sum of critical delays than the other solutions in the set of solutions. Pruning the at least one solution from the set of solutions.

BACKGROUND

The present disclosure relates to optimizing timing in an integratedcircuit, and more specifically, to determining buffer placement tooptimize timing in an integrated circuit.

Operational clock frequency and timing are often key considerations inthe design of high-performance integrated circuits. However, as they arenot the only considerations that go into circuit design, there areinevitably instances where timing is negatively impacted by other designconsiderations. One such instance is related to buffering the circuit.Buffers can serve as isolators (e.g. to control data transmission) oramplifiers (for weak signals), but also act to delay the signal and maythus impede proper circuit timing.

SUMMARY

According to embodiments of the present disclosure, a method foroptimizing timing in a VLSI circuit comprising is described.

The method may begin by generating a set of buffer solutions. The methodmay proceed by determining a most critical delay and a sum of criticaldelays for each solution in the set of solutions. A relationship betweenthe most critical delay and the sum of critical delays may then bequantified for each solution.

Each solution's relationship between the most critical delay and the sumof critical delays may be compared to the relationship between the mostcritical delay and the sum of critical delays of each other solution inthe set of solutions. Based on the comparing of each solution'srelationship to the relationship of each other solution in the set ofsolutions, at least one solution in the set of solutions may beidentified to have a worse relationship between the most critical delayand the sum of critical delays than the other solutions in the set ofsolutions. The at least one solution may then be pruned from the set ofsolutions.

A computing system and computer program product can embody the methodand structures of the disclosure. The computing system can comprise anetwork, a memory configured to store buffer solutions and solutioncharacteristics, and a processor in communication with the memory. Thecomputing system can be configured to perform the method.

The above summary is not intended to describe each illustratedembodiment or every implementation of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings included in the present application are incorporated into,and form part of, the specification. They illustrate embodiments of thepresent disclosure and, along with the description, serve to explain theprinciples of the disclosure. The drawings are only illustrative ofcertain embodiments and do not limit the disclosure.

FIGS. 1A and 1B depicts an illustration of how consideration of all sinknegative slacks may impact buffer solution selection.

FIG. 2 depicts a method for optimizing timing in a VLSI circuit,according to embodiments of the present disclosure.

FIG. 3 depicts another method for optimizing timing in a VLSI circuit,according to embodiments of the present disclosure.

FIG. 4 depicts a simple example of a system for executing the methodsdescribed herein, according to embodiments of the present disclosure.

FIG. 5 depicts a block diagram of an exemplary design flow used, forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture.

FIG. 6 is a high-level block diagram of an example computer system thatmay be used in implementing one or more of the methods described herein,in accordance with embodiments of the present disclosure.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

Aspects of the present disclosure relate to optimizing timing in a verylarge scale integrated circuit (VLSI circuit), and more particularaspects relate to methods and systems for determining an optimal buffersolution for a VLSI circuit. While the present disclosure is notnecessarily limited to such applications, various aspects of thedisclosure may be appreciated through a discussion of various examplesusing this context.

VLSI circuits, and integrated circuits in general, are often designed bylarge teams of engineers and aided by highly specialized computers andcomputer programs. Due to the complex interrelations among an integratedcircuit's many parts, computer assistance plays a fundamental andnecessary role in their design. Improvement of these computerized toolsmay be continuously necessary to move the design of integrated circuitsfurther forward.

Circuit timing is a major consideration in the design ofhigh-performance integrated circuits, such as VLSI circuits. Buffers maybe necessitated by other design considerations of the circuit, and mayin some configurations cause signal delays beyond the circuit'stolerances. As such, buffering solutions for integrated circuits aregenerally concerned with limiting the slack at a particular node withthe worst slack (the most critical delay) in the circuit. While in somecases this may be a sufficient consideration to identify a satisfactorysolution with respect to timing concerns, focusing on the most criticaldelay (MCD) without consideration of the other negative slacks in thecircuit can often result in driving up the total negative slack in thecircuit. Referring now to FIGS. 1A and 1B, an example is presented ofthe negative impacts on buffer placement that may result from onlyconsidering a circuit's worst slack.

FIG. 1A depicts an example circuit with a simple buffer solutionresulting in a MCD of −30 and a total negative slack, or figure of merit(FOM), of −70. FIG. 1B depicts the same example circuit with the buffersolution modified to reduce the worst slack from −30 to −27. However, asa result, the FOM of the circuit increased from −70 to −107. This is adisproportionate cost for the marginal improvement in the worst slack,and likely would not result in overall improved performance from thesolution in FIG. 1A.

Although it is relatively simple to demonstrate that there is in fact arelationship between the MCD and the FOM of a circuit, understanding therelationship fully enough to manipulate it for optimizing the circuitdesign is more complex. The present disclosure describes a method andsystem for optimizing the timing in a circuit by accounting for thecircuit's FOM when assessing buffering solutions. By considering theimpact of a buffering solution on the circuit's total negative slack,and not only the node with the single worst slack, more optimalbuffering solutions may be identified.

Referring now to FIG. 2, a method 200 for optimizing timing in a VLSIcircuit is described, according to embodiments of the presentdisclosure. The method 200 may in some instances be referred to as a“bottom up” method of pruning buffer solutions.

The method 200 may begin by traversing the circuit from any sinks in thecircuit to the source, as in operation 205. Traversing the circuit maygenerally refer to timing analysis of a signal as it travels through thecircuit. Traversal of the circuit may generally involve identifying thearrival time of the signal for each sink in the circuit, and determiningany associated slack. In embodiments, slack may be determined for eachnode of the circuit. Slack may generally be calculated as the differencebetween a node's required time and the measured arrival time. Inembodiments, operation 205 may involve static timing analysis, dynamictiming analysis, circuit simulation, gate level simulation, etc.

At operation 210, a set of buffer solutions may be generated based onthe traversal. A set of solutions may generally include any number ofdifferent buffer arrangements possible for a particular circuit.

At operation 215, a solution may be characterized. The characterizationmay vary among embodiments, according to circuit design parameters anddesired evaluation criteria. In embodiments, the characterization mayinvolve associating a slack value to each node in a circuit. Inembodiments, characterizing the solution may involve determining otherslack characteristics, such as a solution's load cap (the circuit'smaximum load, e.g. current), buffer cost (buffer capacitance is relatedto power consumption, e.g. wire length and buffer area), etc.

In embodiments, the characterization may allow for a determination ofthe worst slack or the most critical delay (MCD), or the node or pathwith the single largest negative slack in the circuit for a givensolution, as in operation 235. The MCD may be identified by calculatingthe delay at each node, where the slack for a given node (SLACK_(n)) isfound by subtracting the delay between the sink and the node(DELAY_(n.s)) from the required arrival time for the sink (RAT_(s)):SLACK_(n)=RAT_(s)−DELAY_(n,s)Since slack may encompass any arrival time that deviates from therequired time, whether late or early, this calculation provides thatlate signals will result in negative values. In embodiments, negativeslacks may be defined by setting a slack threshold t:SLACK_(n)′=max(t−SLACK_(n),0)so that all positive slack values are treated as zero and do notcontribute to the calculation. Once identified, these negative slacks,or critical delays, may be sorted by value. Once sorted, the delay withthe greatest value (i.e. the MCD) may be identified.

In embodiments, the characterization may allow a determination of thetotal critical delay in the circuit, or the FOM, as in operation 240.The FOM may generally be calculated by taking the sum of all criticaldelays, or negative slacks, in the circuit. In some embodiments, thismay be found by calculating the slack for each node and dismissing thoseslacks above a certain threshold (e.g., zero). The remaining (in thisexample, negative) slacks may then be summed to yield the FOM for thecircuit.

In embodiments, the method may proceed to quantifying a relationshipbetween the most critical delay and the sum of critical delays, as inoperation 245. Though solutions may be assessed using the raw values forthe MCD and the FOM for each solution, by quantifying a relationshipbetween the two, the tradeoff between these two possible merits of asolution may be better understood and more optimum solutions found. Therelationship between the MCD and the FOM (referred to herein as “therelationship”) may generally be found by treating the traversal of thecircuit as a vector value and taking the p-norm of the circuit's FOM.

If the traversal of the circuit is treated as a vector x, and the slacksof the nodes are filtered so that each of x₁, x₂, . . . , x_(n) is thenegative slack of a node n, then:x=(x ₁ ,x ₂ , . . . ,x _(n))where x is the FOM. The p-norm of x may then be found as:∥x∥p=(Σ^(n) _(i=1) |x _(i)|^(p))^(1/p)where if p=1 then the p-norm of the FOM, or the PFOM, yields:∥x∥1=Σ^(n) _(i=1) |x _(i)|which, similar to FOM, is a sum of the considered slacks. Conversely, ifp=∞, then the PFOM yields:∥x∥∞=max(|x ₁ |,|x ₂ |, . . . ,|x _(n)|)which, similar to the MCD, is the largest single value contributing tothe vector. Thus, by varying p between 1 and ∞, the PFOM allows for therelationship between the MCD and the FOM to be numerically evaluated.This allows for easier visualization and balancing of the two metrics,and thus selection of more optimal solutions. Slack values used tocalculate the PFOM may generally be calculated from the source:SLACK_(sink)=RAT_(sink)−DELAY_(node,sink)−DELAY_(source,node)rather than per node as they may be when calculating slack fordetermining the MCD.

After quantifying the relationship for a given solution, a review may beexecuted to ensure that each solution has been fully characterized, asin decision block 250. If each solution has not yet been characterized,then the method may return to operation 215 and characterize anothersolution. If each solution in the set of solutions has beencharacterized, then the method may proceed to operation 220.

In operation 220, the relationship between the MCD and the FOM for eachsolution is compared to the relationship between the MCD and the FOM foreach of the other solutions in the set of solutions. In someembodiments, the comparing may be carried out by ranking solutions byvalue. The value used to determine ranking may vary among embodiments.

In operation 225, at least one solution in the set of solutions isidentified to have a worse relationship between the most critical delayand the sum of critical delays than the other solutions in the set ofsolutions, as a result of the comparing. What constitutes a “worse”relationship may generally be defined per circuit, based on thecircuit's design parameters. In embodiments, what constitutes a worserelationship may be determined by a user. In embodiments, a worserelationship may be identified if the value of a solution's relationshipdeviates from a set of design parameters by a greater margin than thevalue of another solution's relationship. In embodiments, whatconstitutes a worse relationship may be calculated based on the range ofPFOM values derived. In embodiments, a worse relationship may generallybe a higher PFOM value, i.e. those PFOM values representing a morepronounced tradeoff between the MCD and FOM.

In embodiments, the comparison of operation 220 and identifying ofoperation 225 may consider more than the relationship between the MCDand the FOM. In some embodiments, the raw value of the MCD or the FOMmay be considered. In some embodiments, both the raw score MCD and FOMmay be considered. In some embodiments, each solution may be furthercharacterized by other factors, e.g. a buffer cost and/or a load cap. Insuch embodiments, solutions could be identified to have a worse totalcharacterization, or a worse partial characterization, based on acomparison of each factor considered.

In operation 230, the at least one solution identified in operation 225is pruned (i.e., removed from consideration) from the set of solutions.In embodiments, solutions may be pruned from the set of solutions untila target number of solutions remains. In embodiments, the target numberof solutions remaining may be a single solution. In embodiments, thesize of the target number of solutions may be predetermined by a set ofdesign parameters. In embodiments, the size of the target set ofsolutions may be determined as a result of the comparing of eachsolution's relationship to the relationship of each other solution inthe starting set of solutions.

In embodiments considering multiple factors, solutions may not be prunedunless each of the factors compared and considered is found to be worsethan each of the factors of at least one other solution in the set ofsolutions. For example, in an embodiment where each solution ischaracterized by a load cap and a buffer cost in addition to therelationship between the MCD and the FOM, a solution may be pruned onlyif each of its load cap, buffer cost, and relationship is found to beworse than each of the load cap, buffer cost, and relationship ofanother solution in the set of solutions. Generally, a lower load capand a higher buffer cost would be considered to be worsecharacteristics, so, in this example, a solution would be pruned only ifit has a lower load cap than another solution, a higher buffer cost thanthe same other solution, and a worse relationship between the MCD andthe FOM than the same other solution.

In embodiments, the remaining solutions may be displayed on a userinterface. Following pruning, a user may further prune solutions fromthe target number of remaining solutions. Following pruning, theremaining solutions may be further evaluated, for example, by repeatedexecution of operation 220, and being comparatively analyzed against oneanother. In some embodiments, solutions may be submitted as part of acircuit design for fabrication, following pruning. In embodiments,pruning and operation 225 may be foregone and a best PFOM solutionselected without first pruning worse solutions from the set ofsolutions. In embodiments where pruning is foregone, runtime overhead ofmethod 200 may be reduced, but less improvement in FOM may also occur.

Referring now to FIG. 3, a method 300 for optimizing timing in a VLSIcircuit is described, according to embodiments of the presentdisclosure. The method 300 may in some instances be referred to as an“at source” method of pruning solutions.

The method 300 may begin by traversing the circuit from any sinks to thesource, as in operation 305. Traversing the circuit may generally referto timing a signal as it travels through the circuit. Signals arrivingbefore or after a required time may be said to have “slack,” withnegative slack encompassing any signal arriving late.

At operation 310, a set of buffer solutions may be generated based onthe traversal. A set of solutions may generally include any number ofdifferent buffer arrangements possible for a particular circuit.

At operation 315, each solution in the generated set of solutions ischaracterized. In embodiments, solutions may be characterized with eachof a buffer cost, an MCD, and an FOM.

At operation 320, the relationship between the MCD and the FOM isquantified for each solution to yield a PFOM. In some embodiments, itmay be advantageous to adjust the definition of PFOM for the at sourcemethod:PFOM=MCD+FOM/pThis may provide a PFOM more consistent with the greater weight theat-source method gives to the MCD, as compared with the bottom-upmethod.

In operation 325, each solution is sorted according to buffer cost. Inembodiments, solutions with buffer costs exceeding permissible designparameters may be discarded at this operation as well.

At decision block 330, each set of solutions for a given buffer cost areassessed to determine the best MCD, i.e. the highest (or least negative)MCD, that is achievable for a given buffer cost. Solutions found to havethe highest MCD for a given buffer cost are sorted to a first list, asin operation 350. In embodiments, multiple solutions in a set may havethe same best MCD for a given buffer cost. In embodiments, the firstlist may comprise solutions with the best MCD for a range of buffercosts.

Once the first list is populated, the knee point value of the solutionsin the first list may be identified and the associated MCD value set asthe critical delay cap, as in operation 355. In embodiments, the kneepoint may be found by plotting the solutions from the first list, forexample, according to MCD and buffer cost. The knee point may beidentified as a point of inflection on the plot of the first list. Inembodiments, the knee point may encompass several solutions with anarrow range of MCD values. The MCD value, or small range of values,identified at the knee point may be set at the critical delay cap. Inembodiments, the critical delay cap may represent the best achievableMCD for a set of solutions for a given set of design parameters. Thecritical delay cap may be used to determine a threshold for pruningsolutions from the set of solutions, and may be used in conjunction withthe PFOM values of the solutions in the set of solutions.

If, at decision block 330, a solution is found to not have a best MCDfor its buffer cost, the solution may be discarded, as in operation 335.

At decision block 340, each set of solutions for a given buffer cost areassessed to determine the best PFOM achievable for a given buffer cost.In embodiments, the best PFOM may be determined according to designparameters associated with the circuit traversed in operation 305. Inembodiments, the best PFOM may be determined according to a range ofvalues input by a user, or determined by a system according to the rangePFOM values calculated in operation 320.

Solutions found to have the best PFOM for a given buffer cost are sortedto a second list, as in operation 345. In embodiments, multiplesolutions in a set may have the same best PFOM value for a given buffercost. In embodiments, the second list may comprise solutions with thebest PFOM for a range of buffer costs. Similar to the first list, thesecond list may, in embodiments, have a knee point PFOM value or rangeof values, found at an inflection point of the solutions when plottedagainst buffer cost and PFOM value. In embodiments, a solution may havea worse characterization if the solution's relationship between the mostcritical delay and the sum of critical delays comprises a higher valuethan a relationship between the most critical delay and the sum ofdelays of at least one other solution of the set of solutions.

In embodiments, decision block 340 may be executed in parallel withdecision block 330. In embodiments, decision blocks 330 and 340 may beexecuted in sequence. Both decision block 330 and 340 may be executedwith the full set of solutions generated in operation 310 and sorted in325. In embodiments where decision blocks 330 and 340 are executed insequence, either decision block 330 or 340 may be used to pare down thesolution set. For example, if decision block 340 is executed subsequentto decision block 330, decision block 340 may be executed with a set ofsolutions which does not contain those solution discarded in the processof executing decision block 330.

If, at decision block 340, a solution is found to not have a best PFOMfor its buffer cost, the solution may be discarded, as in operation 335.

At operation 360, the second list may be pruned using the threshold setwith the critical delay cap from operation 355, resulting in a “reducedsecond list.” Pruning the second list may involve discarding anysolution with an MCD value exceeding the critical delay cap by aparticular margin. In embodiments, the margin may be predeterminedaccording to a set of design parameters, or calculated based on therange of MCD values determined at operation 315 or the range of MCDvalues present in the second list. In embodiments, the threshold may bean upper threshold, or a range representing an upper and a lowerthreshold. In embodiments with both an upper and a lower threshold, therange delineated by the threshold may be determined according to a setof design parameters, as well as the critical delay cap.

Following pruning, the reduced second list may be replotted and the kneepoint adjusted as appropriate to accommodate the reduction in the listvalues. In embodiments where the knee point of the second listrepresents a range, adjusting the knee point may involve narrowing therange.

At operation 365, a solution, or target set of solutions, may beselected from the adjusted knee point of the second list. Inembodiments, a set of final solutions may be selected from closeproximity to the knee point. Close proximity to the knee point may bedetermined according to a set of design parameters associated with thecircuit traversed in operation 305. The size of the set of finalsolutions may also be determined according to the set of designparameters. In embodiments, pruning, and operation 360, may be foregoneand a best PFOM solution selected without first pruning worse solutionsfrom the set of solutions. In embodiments wherein pruning is foregone,runtime overhead of method 300 may be reduced, but less improvement inFOM may also be expected.

At operation 370, one or more of the solutions selected at operation 365may be communicated for circuit design and/or fabrication. Inembodiments, the selected solutions may be displayed on a userinterface.

Methods 200, 300 described above may be executed by a system. Referringnow to FIG. 4, a simple example of a system 400 for executing methods200, 300 is depicted, according to embodiments of the presentdisclosure. In embodiments, system 400 may comprises a portion of anelectronic design automation system (ECAD system).

Operation of system 400 may utilize design parameters 405, which may beset by user input or determined by an external system (e.g. anotherportion of an ECAD system). Design parameters 405 may compriseparameters for digital or analog IC design. In embodiments, system 400may be particularly for digital design, with a particular focus onrouting clock and timing signals efficiently. Design parameters 405 mayinclude functional and logical design determinations for the desiredresulting circuit design.

A circuit template 410, or multiple circuit templates 410, may begenerated according to design parameters 405. In embodiments, generationof the circuit template may be integral to system 400, as in thisexample, or executed by an external system and the circuit template 410input independently or with design parameters 405.

A solution generator 420 may receive input from the circuit template 410and design parameters 405 for the generation of a number of buffersolutions 422. In embodiments, generation of buffer solutions 422 may beintegrated into system 400, as in this example, or executed by anexternal system and the buffer solutions 422 input to system 400.

Buffer solutions 422 may be assessed by a timing module 415 (e.g. statictiming analysis) according to input from design parameters 405.Assessment by timing module 415 may include slack determination for eachof buffer solutions 422.

Characterizing module 425 may receive input from buffer solutions 422and timing module 415 and assign characteristics to buffer solutions 422accordingly. Characterizing module 425 may assign each buffer solution422 an associated load cap, buffer cost, MCD, and FOM. Characteristicsmay be assigned to buffer solutions 422 individually or in groups,according to the desired design parameters 405. In embodiments, anycombination of characteristic may be determined for buffer solutions422. Characterizing module 425 may also be responsible to quantifyingthe relationship between the MCD and FOM for each solution 422. Oncebuffer solutions 422 are characterized, characterized solutions 428 maybe output by characterizing module 425.

Characterized solutions 428 may then be compared to one another bycomparing/sorting module 430 and sorted appropriately. Characterizedsolutions 428 may generally be ranked by comparing/sorting module 430 toassist in identifying best and worst solutions according to designparameters 405. Comparing/sorting module 430 may generally output to apruning/selecting module 440. In embodiments carrying out an at-sourcemethod, such as method 300, the comparing/sorting module 430 may alsooutput to a graphing module 435.

Graphing module 435 may receive sorted solutions from comparing/sortingmodule 430 and may plot them to identify a knee point, as for anat-source method (e.g. method 300). Graphing module 435 may also receivea pruned set of sorted solutions from pruning/selecting module 440, toplot and identify the knee point. In embodiments, graphing module 435may also act to adjust the knee point on a set of solutions that hasbeen previously graphed, but pruned since it was last graphed.

Pruning/selecting module 440 may receive output from comparing/sortingmodule 430 and, in embodiments, from graphing module 435.Pruning/selecting module 440 may prune and/or discard solutionsidentified as poor, or worse, by comparing/sorting module 430 and/orgraphing module 435. In embodiments, pruning/selecting module 440 mayselect solutions identified as options for a given set of designparameters 405 by comparing/sorting module 430 or graphing module 435.Solutions selected by the pruning/selecting module 440 may comprise thedesign output of system 400. In embodiments, the design output of system400 may be further processed to yield a physical circuit design.

FIG. 5 shows a block diagram of an exemplary design flow 500 used forexample, in semiconductor IC logic design, simulation, test, layout, andmanufacture. Design flow 500 includes processes, machines and/ormechanisms for processing design structures or devices to generatelogically or otherwise functionally equivalent representations of thedesign structures and/or devices described above and output by methods200, 300. The design structures processed and/or generated by designflow 500 may be encoded on machine-readable transmission or storagemedia to include data and/or instructions that when executed orotherwise processed on a data processing system generate a logically,structurally, mechanically, or otherwise functionally equivalentrepresentation of hardware components, circuits, devices, or systems.Machines include, but are not limited to, any machine used in an ICdesign process, such as designing, manufacturing, or simulating acircuit, component, device, or system. For example, machines mayinclude: lithography machines, machines and/or equipment for generatingmasks (e.g. e-beam writers), computers or equipment for simulatingdesign structures, any apparatus used in the manufacturing or testprocess, or any machines for programming functionally equivalentrepresentations of the design structures into any medium (e.g. a machinefor programming a programmable gate array).

Design flow 500 may vary depending on the type of representation beingdesigned. For example, a design flow 500 for building an applicationspecific IC (ASIC) may differ from a design flow 500 for designing astandard component or from a design flow 500 for instantiating thedesign into a programmable array, for example a programmable gate array(PGA) or a field programmable gate array (FPGA) offered by Altera® Inc.or Xilinx® Inc.

FIG. 5 illustrates multiple such design structures including an inputdesign structure 520 that is preferably processed by a design process510. Design structure 520 may be a logical simulation design structuregenerated and processed by design process 510 to produce a logicallyequivalent functional representation of a hardware device. Designstructure 520 may also or alternatively comprise data and/or programinstructions that when processed by design process 510, generate afunctional representation of the physical structure of a hardwaredevice. Whether representing functional and/or structural designfeatures, design structure 520 may be generated using electroniccomputer-aided design (ECAD) such as implemented by a coredeveloper/designer. When encoded on a machine-readable datatransmission, gate array, or storage medium, design structure 520 may beaccessed and processed by one or more hardware and/or software moduleswithin design process 510 to simulate or otherwise functionallyrepresent an electronic component, circuit, electronic or logic module,apparatus, device, or system. As such, design structure 520 may comprisefiles or other data structures including human and/or machine-readablesource code, compiled structures, and computer-executable codestructures that when processed by a design or simulation data processingsystem, functionally simulate or otherwise represent circuits or otherlevels of hardware logic design. Such data structures may includehardware-description language (HDL) design entities or other datastructures conforming to and/or compatible with lower-level HDL designlanguages such as Verilog and VHDL, and/or higher level design languagessuch as C or C++.

Design process 510 preferably employs and incorporates hardware and/orsoftware modules for synthesizing, translating, or otherwise processinga design/simulation functional equivalent of the components, circuits,devices, or logic structures output by methods 200, 300 to generate aNetlist 580 which may contain design structures such as design structure520. Netlist 580 may comprise, for example, compiled or otherwiseprocessed data structures representing a list of wires, discretecomponents, logic gates, control circuits, I/O devices, models, etc.that describes the connections to other elements and circuits in anintegrated circuit design. Netlist 580 may be synthesized using aniterative process in which netlist 580 is resynthesized one or moretimes depending on design specifications and parameters for the device.As with other design structure types described herein, netlist 580 maybe recorded on a machine-readable data storage medium or programmed intoa programmable gate array. The medium may be a non-volatile storagemedium such as a magnetic or optical disk drive, a programmable gatearray, a compact flash, or other flash memory. Additionally, or in thealternative, the medium may be a system or cache memory, buffer space,or electrically or optically conductive devices and materials on whichdata packets may be transmitted and intermediately stored via theInternet, or other networking suitable means.

Design process 510 may include hardware and software modules forprocessing a variety of input data structure types including Netlist580. Such data structure types may reside, for example, within libraryelements 530 and include a set of commonly used elements, circuits, anddevices, including models, layouts, and symbolic representations, for agiven manufacturing technology (e.g., different technology nodes, 32 nm,45 nm, 90 nm, etc.). The data structure types may further include designspecifications 540, characterization data 550, verification data 560,design rules 570, and test data files 585 which may include input testpatterns, output test results, and other testing information. Designprocess 510 may further include, for example, standard mechanical designprocesses such as stress analysis, thermal analysis, mechanical eventsimulation, process simulation for operations such as casting, molding,and die press forming, etc. One of ordinary skill in the art ofmechanical design can appreciate the extent of possible mechanicaldesign tools and applications used in design process 510 withoutdeviating from the scope and spirit of the invention. Design process 510may also include modules for performing standard circuit designprocesses such as timing analysis, verification, design rule checking,place and route operations, etc.

Design process 510 employs and incorporates logic and physical designtools such as HDL compilers and simulation model build tools to processdesign structure 520 together with some or all of the depictedsupporting data structures along with any additional mechanical designor data (if applicable), to generate a second design structure 590.Design structure 590 resides on a storage medium or programmable gatearray in a data format used for the exchange of data of mechanicaldevices and structures (e.g. information stored in a IGES, DXF,Parasolid XT, JT, DRG, or any other suitable format for storing orrendering such mechanical design structures). Similar to designstructure 520, design structure 590 preferably comprises one or morefiles, data structures, or other computer-encoded data or instructionsthat reside on transmission or data storage media and that whenprocessed by an ECAD system generate a logically or otherwisefunctionally equivalent form of one or more of the designs output bymethods 200, 300 above. In one embodiment, design structure 590 maycomprise a compiled, executable HDL simulation model that functionallysimulates the devices that designs output by method 200, 300 may yield.

Design structure 590 may also employ a data format used for the exchangeof layout data of integrated circuits and/or symbolic data format (e.g.information stored in a GDSII (GDS2), GL1, OASIS, map files, or anyother suitable format for storing such design data structures). Designstructure 590 may comprise information such as, for example, symbolicdata, map files, test data files, design content files, manufacturingdata, layout parameters, wires, levels of metal, vias, shapes, data forrouting through the manufacturing line, and any other data required by amanufacturer or other designer/developer to produce a device orstructure as described above. Design structure 590 may then proceed to astage 595 where, for example, design structure 590: proceeds totape-out, is released to manufacturing, is released to a mask house, issent to another design house, is sent back to the customer, etc.

Referring now to FIG. 6, shown is a high-level block diagram of anexample computer system (i.e., computer) 600 that may be used inimplementing one or more of the methods or modules, and any relatedfunctions or operations, described herein (e.g., using one or moreprocessor circuits or computer processors of the computer), inaccordance with embodiments of the present disclosure. In someembodiments, the major components of the computer system 600 maycomprise one or more CPUs 602, a memory subsystem 604, a terminalinterface 612, an I/O (Input/Output) device interface 614, a storageinterface 616, and a network interface 618, all of which may becommunicatively coupled, directly or indirectly, for inter-componentcommunication via a memory bus 603, an I/O bus 608, and an I/O businterface unit 610.

The computer system 600 may contain one or more general-purposeprogrammable central processing units (CPUs) 602A, 602B, 602C, and 602D,herein generically referred to as the CPU 602. In some embodiments, thecomputer system 600 may contain multiple processors typical of arelatively large system; however, in other embodiments the computersystem 600 may alternatively be a single CPU system. Each CPU 602 mayexecute instructions stored in the memory subsystem 604 and may compriseone or more levels of on-board cache.

In some embodiments, the memory subsystem 604 may comprise arandom-access semiconductor memory, storage device, or storage medium(either volatile or non-volatile) for storing data and programs. In someembodiments, the memory subsystem 604 may represent the entire virtualmemory of the computer system 600, and may also include the virtualmemory of other computer systems coupled to the computer system 600 orconnected via a network. The memory subsystem 604 may be conceptually asingle monolithic entity, but, in some embodiments, the memory subsystem604 may be a more complex arrangement, such as a hierarchy of caches andother memory devices. For example, memory may exist in multiple levelsof caches, and these caches may be further divided by function, so thatone cache holds instructions while another holds non-instruction data,which is used by the processor or processors. Memory may be furtherdistributed and associated with different CPUs or sets of CPUs, as isknown in any of various so-called non-uniform memory access (NUMA)computer architectures. In some embodiments, the main memory or memorysubsystem 604 may contain elements for control and flow of memory usedby the CPU 602. This may include a memory controller 605.

Although the memory bus 603 is shown in FIG. 6 as a single bus structureproviding a direct communication path among the CPUs 602, the memorysubsystem 604, and the I/O bus interface 610, the memory bus 603 may, insome embodiments, comprise multiple different buses or communicationpaths, which may be arranged in any of various forms, such aspoint-to-point links in hierarchical, star or web configurations,multiple hierarchical buses, parallel and redundant paths, or any otherappropriate type of configuration. Furthermore, while the I/O businterface 610 and the I/O bus 608 are shown as single respective units,the computer system 600 may, in some embodiments, contain multiple I/Obus interface units 610, multiple I/O buses 608, or both. Further, whilemultiple I/O interface units are shown, which separate the I/O bus 608from various communications paths running to the various I/O devices, inother embodiments some or all of the I/O devices may be connecteddirectly to one or more system I/O buses.

In some embodiments, the computer system 600 may be a multi-usermainframe computer system, a single-user system, or a server computer orsimilar device that has little or no direct user interface, but receivesrequests from other computer systems (clients). Further, in someembodiments, the computer system 600 may be implemented as a desktopcomputer, portable computer, laptop or notebook computer, tabletcomputer, pocket computer, telephone, smart phone, mobile device, or anyother appropriate type of electronic device.

It is noted that FIG. 6 is intended to depict the representative majorcomponents of an exemplary computer system 600. In some embodiments,however, individual components may have greater or lesser complexitythan as represented in FIG. 6, components other than or in addition tothose shown in FIG. 6 may be present, and the number, type, andconfiguration of such components may vary.

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, configuration data for integrated circuitry, oreither source code or object code written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Smalltalk, C++, or the like, and procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer readable program instructions may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider). In some embodiments, electronic circuitry including,for example, programmable logic circuitry, field-programmable gatearrays (FPGA), or programmable logic arrays (PLA) may execute thecomputer readable program instructions by utilizing state information ofthe computer readable program instructions to personalize the electroniccircuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the Figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

The descriptions of the various embodiments of the present disclosurehave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A computer program product for optimizing timingin a VLSI circuit, the computer program product comprising a computerreadable storage medium having program instructions embodied therewith,wherein the computer readable storage medium is not a transitory signalper se, the program instructions executable by a processor to perform amethod comprising: generating a set of buffer solutions for the VLSIcircuit; determining a most critical delay and a sum of delays for eachsolution of the set of solutions; quantifying, for each solution, arelationship between the most critical delay and the sum of delays;comparing each solution's relationship to the relationship of each othersolution in the set of solutions; identifying, based on the comparing ofeach solution's relationship to the relationship of each other solutionin the set of solutions, at least one solution of the set of solutionsto have a worse relationship between the most critical delay and the sumof delays than the other solutions in the set of solutions; and pruningthe at least one solution from the set of solutions.
 2. The computerprogram product of claim 1, wherein the method further comprisescontinuing to prune solutions from the set of solutions until a targetnumber of solutions remains.
 3. The computer program product of claim 1,wherein the method further comprises submitting at least one remainingsolution to an integrated circuit design process.
 4. The computerprogram product of claim 1, wherein the method further comprisesdisplaying at least one solution remaining in the set of the solutionson a user interface.
 5. The computer program product of claim 1, whereinquantifying a relationship between the most critical delay and the sumof critical delays comprises calculating a p-norm of the sum of delays.6. The computer program product of claim 5, wherein a solution has aworse relationship between the most critical delay and the sum ofcritical delays when the solution's p-norm of the sum of delayscomprises a higher value than a p-norm of the sum of delays of at leastone other solution of the set of solutions.
 7. The computer programproduct of claim 5, wherein the set of solutions is created according toa set of design parameters; and wherein a solution has a worserelationship between the most critical delay and the sum of delays whenthe solution's relationship is associated with a value which deviatesfrom the design parameters by a greater margin than a value associatedwith the relationship between the most critical delay and the sum ofcritical delays of at least one other solution of the set of solutions.8. The computer program product of claim 1, wherein the method furthercomprises determining a load cap and a buffer cost for each solution ofthe set of solutions.
 9. The computer program product of claim 8,wherein the method further comprises comparing each solution's load capand buffer cost with the load cap and buffer cost of each other solutionin the set of solutions, wherein each solution has a characterizationand the characterization comprises the solution's load cap, buffer cost,and relationship between the most critical delay and the sum of criticaldelays; and wherein the identifying, based on the comparing of eachsolution's relationship to the relationship of each other solution inthe set of solutions, at least one solution of the set of solutions tohave a worse relationship than the other solutions in the set ofsolutions further comprises identifying, based on the comparing of eachsolution's load cap and buffer cost with the load cap and buffer cost ofeach other solution in the set of solutions, at least one solution ofthe set of solutions to have a worse characterization than the othersolutions in the set of solutions.
 10. The computer program product ofclaim 9, wherein a solution has a worse characterization if thesolution's load cap has a lower value and the solution's buffer cost hasa higher value than a load cap and a buffer cost of at least one othersolution of the set of solutions.
 11. The computer program product ofclaim 1, wherein the circuit comprises one or more nodes and the mostcritical delay for each solution is found by calculating the delay ateach node and identifying the node with the largest delay.
 12. Thecomputer program product of claim 1, wherein the circuit comprises asource and one or more sinks, and wherein the sum of critical delays fora solution is calculated using a measurement of each critical delay inthe solution taken from the sink to the source.
 13. The computer programproduct of claim 1, further comprising: fabricating, after the pruningthe at least one solution from the set of solutions, the VLSI circuitbased on a first solution in the set of solutions.
 14. A computerprogram product for optimizing timing in a VLSI circuit, the computerprogram product comprising a computer readable storage medium havingprogram instructions embodied therewith, wherein the computer readablestorage medium is not a transitory signal per se, the programinstructions executable by a processor to perform a method comprising:generating a set of buffer solutions for the VLSI circuit; determining amost critical delay, a sum of critical delays, and a buffer cost foreach solution of the set of solutions; quantifying, for each solution, arelationship between the most critical delay and the sum of criticaldelays; creating a first list containing each solution with a highestmost critical delay for a given buffer cost, the first list having afirst knee point, the first knee point having an associated criticaldelay cap; determining a threshold based on the critical delay capassociated with the first knee point; creating a second list containingeach solution with a best relationship between the most critical delayand the sum of critical delays for a given buffer cost, the second listhaving a second knee point; pruning the second list based on thethreshold; adjusting, in response to pruning the second list based onthe threshold, the second knee point; selecting at least one solutionfrom the adjusted second knee point.
 15. The computer program product ofclaim 14, wherein selecting at least one solution from the second kneepoint comprises selecting a set of final solutions in proximity to theknee point.
 16. The computer program product of claim 15, wherein theset of final solutions is sized according to a set of design parameters.17. The computer program product of claim 14, wherein the method furthercomprises communicating the at least one selected solution for circuitfabrication.
 18. The computer program product of claim 14, wherein eachsolution is further characterized with a load cap.
 19. The computerprogram product of claim 18, wherein a solution has a worse relationshipif the solution's relationship between the most critical delay and thesum of delays comprises a higher value than a relationship between themost critical delay and the sum of delays of at least one other solutionof the set of solutions.
 20. The computer program product of claim 19,wherein the set of solutions is created according to a set of designparameters; and wherein a solution has a worse relationship if thesolution's relationship between the most critical delay and the sum ofdelays is associated with a value which deviates from the designparameters by a greater margin than a value associated with therelationship between the most critical delay and the sum of delays of atleast one other solution of the set of solutions.